1. Technical Field
The present disclosure relates to semiconductor packaging, and in particular to leadframe-based packages, to leadframes employed in such packages, and most particularly to flat, leadless type packages and leadframes, such as, e.g., QFN packages.
2. Description of the Related Art
While significant advances have been made in semiconductor packaging, including the development of a very large number of packaging types, the majority of semiconductor devices still employ leadframe based packages. This is due to a number of reasons. In particular, leadframe packages are relatively inexpensive to produce, are well known and understood, the tooling is already in hand or readily available, etc. Additionally, leadframe packages have some advantages over many other packages, including having better thermal transmission characteristics, and more being more robust. As industry demand moves toward smaller packages and higher contact density, leadframe technology continues to evolve, so that even within the general category of leadframe packages, there are many different types. One of the most popular configurations is the flat no-lead-type package, including, for example, the quad flat no-lead (QFN) package. No-lead packages are popular because contact surfaces for I/O are on the back side of the package, so that when in use, they are between the package and the circuit board. This means that the board designer can move the PCB's corresponding contacts into the footprint of the package, which reduces the total real estate occupied by a QFN package, as compared to an otherwise identical package that is configured for surface mount or pin mount.
Additionally, as maximum circuit density in integrated circuits increases, back-end designers are under pressure to keep pace with the demand for higher density circuits by providing packaging that can enable larger numbers of contacts for a given package size. There are a number of physical constraints that interfere with progress in this area, some of which will be discussed below.
FIG. 1A is a plan view of a QFN package 100 according to known art, with the package body 101 rendered transparent to show the configuration of the leadframe and wirebonding. A side view of the device is shown in FIG. 1B. A plurality of bond pads are distributed around a perimeter of the package, arranged in a first row of bond pads 102 and a second row of pads 103 arranged concentrically. Leads 107, which couple bond pads 103 of the second row to a support frame that originally supported them, thread between pairs of the pads 102 of the first row of bond pads to contact the second row of pads. A die paddle 104 is positioned in the center, and a semiconductor die 105 is positioned on the die paddle. Bond wires 109 extend from contact pads 110 on the semiconductor die 105 to bond pads 102 of the first row, while bond wires 108 extend from contact pads 110 on the semiconductor die 105 to bond pads 103 of the second row of pads. There are two conflicting problems that affect the reliability of this device. First, it can be seen that the wires are long, and in many places they are positioned very close to each other as they arc across from the die to respective bond pads. This arrangement is likely prone to shorting between the wires during the encapsulation process because of their excessive length and close proximity. this problem could be reduced somewhat if the bond pads were moved closer to the center of the device so the wires could be shortened, but this raises a different problem.
Referring to FIG. 1B, it can be seen that the leads 107 are thinned between the pads 103 and the outer edge of the device 100. This is done to prevent the leads 107 from unintentionally contacting contact pads on a circuit board that are intended for contact by pads of the first row. As the bond pads are moved toward the center of the device, the leads 107 must be made longer to reach the more distant leads. Given their small dimensions, the leads 107 are not capable of supporting bond pads at any significant distance. On the other hand, they cannot be made either wider or thicker without risking contact with other leads or with underlying circuit board pads.
FIGS. 2A and 2B show another QFN device 200 that has contact pads 202 distributed in a single row about the perimeter, with long leads 203 extending from the contact pads 202 toward the center of the device. This enables the use of shorter bond wires 204 extending from contact pads 207 of the semiconductor die 205, but requires extremely long and thin cantilevered leads 203, as shown in FIG. 2B. This arrangement is again unreliable, and prone to shorting during handling steps prior to encapsulation.
FIGS. 3A and 3B show a known QFN device 300 that includes fingers 304 configured to receive solder bumps 305 of a flip chip 302, placing them in electrical contact with contact pads 303 at the perimeter of the device. As shown in FIG. 3B, the fingers 304 are thinned to raise the bottom surfaces from the bottom of the device, so that the fingers are cantilevered in toward the center of the package. A problem that arises here is that, when the flip chip die 302 is positioned on the fingers 304 by a place-and-pick machine, they have a tendency to flex slightly, then spring back when the machine releases, causing the die 302 to bounce, sometimes bouncing enough to shift the die out of alignment with the fingers 30